1. Field of the Invention
The present invention relates to a piezoelectric actuator drive device capable of driving a piezoelectric actuator.
2. Description of the Related Art
Piezoelectric actuators are well known and widely used in various types of devices. In general, a piezoelectric actuator expands and contracts by electrical charging and discharging in order to control a linear motion of a piston. For example, a piezoelectric actuator controls the linear motion of a piston in a fuel injection device for an internal combustion engine in order to control opening and closing of a valve of a fuel injector. Conventional documents such as Japanese patent laid open publications No. JP 2003-92438 and No JP 2007-205173 disclose such piezoelectric actuators.
There is a known drive device that charges and discharges a piezoelectric actuator in order to expand and contract the piezoelectric actuator. The drive device is comprised of a charging path, a discharging path, a first diode, and a second diode. A DC power source supplies an electric power to a series circuit composed of an inductor and the piezoelectric actuator through the charging path and a charging switch. The charging switch is composed of a metal oxide semiconductor field effect transistor (MOS FET) mounted to the charging path. The electric charge accumulated in the piezoelectric actuator are discharged through the discharging path and a discharging switch connected in parallel to the series circuit. The discharging switch is composed of a MOS FET mounted to the discharging path.
The first diode is connected in parallel to the charging switch so that the cathode of the first diode is connected to the positive electrode of the DC power source. The second diode is connected in parallel to the discharging switch so that the anode of the second diode is connected to the negative electrode of the DC power source.
When receiving a drive signal transferred from an external device, the above drive device for the piezoelectric actuator repeats ON-OFF operation of the charging switch while setting the discharging switch to the OFF state in order to expand the piezoelectric actuator by accumulating electric charge into the piezoelectric actuator.
When receiving no drive signal, the drive device repeats ON-OFF operation of the discharging switch under the OFF state of the charging switch in order to contract the piezoelectric actuator by discharging the electric charge accumulated in the piezoelectric actuator.
During the charging period of the piezoelectric actuator, a charging current is supplied from the DC power source to the piezoelectric actuator through the charging path by turning the charging switch ON. After this, by turning the charging switch OFF, a charging current (namely, a fly wheel current), which flows by the electrical energy charged in the inductor, is supplied to the piezoelectric actuator through the second diode connected in parallel to the discharging switch. The above ON and OFF operations are repeated in order to gradually charge the piezoelectric actuator.
During the discharging period of the piezoelectric actuator, a discharging current is supplied from the positive electrode of the piezoelectric actuator to the discharging path through the inductor by turning the discharging switch ON. After this, the discharging current is supplied from the positive electrode of the piezoelectric actuator to the DC power source through the first diode connected in parallel to the inductor and the charging switch by turning the discharging switch OFF. The electric charge accumulated in the piezoelectric actuator can be recovered to the DC power source by the discharging current. Those operations are repeated in order to discharge the piezoelectric actuator in steps.
In the control of discharging the piezoelectric actuator, the discharging switch is firstly turned ON, and the discharging switch is turned OFF when the detection value of a discharging current from the piezoelectric actuator reaches a peak threshold value. The discharging switch which is OFF is then turned ON when the detection value of the discharging current from the piezoelectric actuator reaches a bottom threshold value. Those operations are repeated.
The conventional document JP 2003-92438 has disclosed the technique to maintain a discharging energy by increasing the above peak threshold value (or a breaking current) according to the elapse of the discharging time. Because the voltage of the piezoelectric actuator is decreased according to the elapse of discharging time, the total amount of energy released as a time product can be kept at a constant by increasing the current of the piezoelectric actuator in order to avoid the fluctuation of the discharging period of time (necessary to discharge).
The conventional document JP 2007-205173 discloses the technique of discharging the electric charge accumulated in the piezoelectric actuator for a predetermined period of time by adjusting the peak threshold value for the discharging control according to the amount of the electric charge of piezoelectric elements in the piezoelectric actuator.
On the other hand, Japanese patent laid open publication No. JP 2008-5649 discloses a conventional technique of charging a piezoelectric actuator to a target amount of energy.
The conventional technique disclosed in JP 2003-92438 only increases a peak threshold value for use in the discharging control for the piezoelectric actuator at a constant gradient. It is accordingly difficult to control an energy discharging change rate (or a rate of energy change during discharging) of the piezoelectric actuator at an optional constant value. In addition, because the energy discharging change rate is changed when an electrostatic capacity of the piezoelectric actuator is changed during the charge of the piezoelectric actuator, the period of time need to discharge is changed.
On the other hand, the conventional technique disclosed in JP 2007-205173 changes the peak threshold value, which is constant to the period of time, in accordance with the amount of electric charge accumulated in the piezoelectric actuator. Because the discharging energy is decreased approximately based on a quadratic function, it is impossible to adjust the energy discharging change rate to an optional constant value.